Auto-correlation of signals has been feasible for some time, but has generally been reserved for off-line applications due to the cost and complexity of the equipment necessary for implementation of a real-time system. Without auto-correlation it has been necessary to transmit a signal of sufficient power to be received at a power level higher than the background noise to insure a high probability of detection and a low false alarm rate, and it also has been necessary to introduce redundancy so that each data bit or message is repeated enough times to insure reliability.